Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. Catalytic cracking, and particularly fluid catalytic cracking (FCC), is routinely used to convert heavy hydrocarbon feedstocks to lighter products, such as gasoline and distillate range fractions. In FCC processes, a hydrocarbon feedstock is injected into the riser section of a FCC unit, where the feedstock is cracked into lighter, more valuable products upon contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator.
It has been recognized that for a fluid catalytic cracking catalyst to be commercially successful, it must have commercially acceptable activity, selectivity, and stability characteristics. It must be sufficiently active to give economically attractive yields, have good selectivity towards producing products that are desired and not producing products that are undesired, and it must be sufficiently hydrothermally stable and attrition resistant to have a commercially useful life.
Exccessive coke and hydrogen is undesirable in commercial catalytic cracking processes. Even small increases in the yields of these products relative to the yield of gasoline can cause significant practical problems. For example, increases in the amount of coke produced can cause undesirable increases in the heat that is generated by burning off the coke during the highly exothermic regeneration of the catalyst. Conversely, insufficient coke production can also distort the heat balance of the cracking process. In addition, in commercial refineries, expensive compressors are used to handle high volume gases, such as hydrogen. Increases in the volume of hydrogen produced, therefore, can add substantially to the capital expense of the refinery.
Improvements in cracking activity and gasoline selectivity of cracking catalysts do not necessarily go hand in hand. Thus, a cracking catalyst can have outstandingly high cracking activity, but if the activity results in a high level of conversion to coke and/or gas at the expense of gasoline the catalyst will have limited utility. Catalytic cracking in current FCC catalysts is attributable to both the zeolite and non-zeolite (e.g. matrix) components. Zeolite cracking tends to be gasoline selective, while matrix cracking tends to be less gasoline selective.
In recent years, the oil refining industry has shifted to processing a larger quantity of residual (resid) and resid-containing feeds due to changes in the price structure and availability of crude oil. Many refiners have been processing at least a portion of residual oil in their units and several now run a full residual oil cracking program. Processing resid feeds can drastically alter yields of valuable products in a negative direction relative to a light feed. Aside from operational optimizations, the catalyst has a large impact on product distribution. Several factors are important to resid catalyst design. It is highly favorable if the catalyst can minimize coke and hydrogen formation, have high stability, and minimize deleterious contaminant selectivity due to metal contaminants in resid feedstocks.
Resid feeds typically contain contaminant metals including Ni, V, Fe, Na, Ca, and others. Resid FCC for converting heavy resid feeds with high Ni and V contaminants constitutes the fastest growing FCC segment globally. Both Ni and V catalyze unwanted dehydrogenation reactions, but Ni is an especially active dehydrogenation catalyst. Ni significantly increases H2 and coke yields. In addition to taking part in unwanted dehydrogenation reactions, V comes with other major concerns as it is highly mobile under FCC conditions and its interaction with the zeolite destroys its framework structure, which manifests itself as increased H2 and coke yields, as well as lower zeolite surface area retention. Even small amounts (e.g., 1-5 ppm) of contaminant metals in the feed cumulatively deposited on the catalyst can result in high H2 and coke yields during FCC operation, if the catalyst does not feature an optimized metals passivation system, which is a major concern for the refining industry.
Since the 1960s, most commercial fluid catalytic cracking catalysts have contained zeolites as an active component. Such catalysts have taken the form of small particles, referred to as microspheres, containing both an active zeolite component and a non-zeolite component in the form of a high alumina, silica-alumina (aluminosilicate) matrix. The active zeolitic component is incorporated into the microspheres of the catalyst by one of two general techniques. In one technique, the zeolitic component is crystallized and then incorporated into microspheres in a separate step. In the second technique, the in situ technique, microspheres are first formed and the zeolitic component is then crystallized in the microspheres themselves to provide microspheres containing both zeolitic and non-zeolitic components. For many years a significant proportion of commercial FCC catalysts used throughout the world have been made by in situ synthesis from precursor microspheres containing kaolin that had been calcined at different severities prior to formation into microspheres by spray drying. U.S. Pat. No. 4,493,902 (“the '902 patent”), incorporated herein by reference in its entirety, discloses the manufacture of fluid cracking catalysts comprising attrition-resistant microspheres containing Y zeolite with faujisite structure, formed by crystallizing sodium Y zeolite in porous microspheres composed of metakaolin and spinel. The microspheres in the '902 patent contain more than about 40%, for example 50-70% by weight Y zeolite. Such catalysts can be made by crystallizing more than about 40% sodium Y zeolite in porous microspheres composed of a mixture of two or more different phases of chemically reactive calcined clay, namely, metakaolin (kaolin calcined to undergo a strong endothermic reaction associated with dehydroxylation) and kaolin clay calcined under conditions more severe than those used to convert kaolin to metakaolin, i.e., kaolin clay calcined to undergo the characteristic kaolin exothermic reaction, sometimes referred to as the spinel form of calcined kaolin. This characteristic kaolin exothermic reaction is sometimes referred to as kaolin calcined through its “characteristic exotherm.” The microspheres containing the two forms of calcined kaolin clay are immersed in an alkaline sodium silicate solution, which is heated, until the desired amount of Y zeolite with faujasite structure is crystallized in the microspheres.
Fluid cracking catalysts which contain silica-alumina or alumina matrices are termed catalysts with “active matrix.” Catalysts of this type can be compared with those containing untreated clay or a large quantity of silica, which are termed “inactive matrix” catalysts. In relation to catalytic cracking, despite the apparent disadvantage in selectivity, the inclusion of aluminas or silica-alumina has been beneficial in certain circumstances. For instance when processing a hydrotreated/demetallated vacuum gas oil (hydrotreated VGO) the penalty in non-selective cracking is offset by the benefit of cracking or “upgrading” the larger feed molecules which are initially too large to fit within the rigorous confines of the zeolite pores. Once “precracked” on the alumina or silica-alumina surface, the smaller molecules may then be selectively cracked further to gasoline material over the zeolite portion of the catalyst. While one would expect that this precracking scenario might be advantageous for resid feeds, they are, unfortunately, characterized as being heavily contaminated with metals such as nickel and vanadium and, to a lesser extent, iron. When a metal such as nickel deposits on a high surface area alumina such as those found in typical FCC catalysts, it is dispersed and participates as highly active centers for the catalytic reactions which result in the formation of contaminant coke (contaminant coke refers to the coke produced discretely from reactions catalyzed by contaminant metals) and hydrogen. This additional coke exceeds that which is acceptable by refiners. Loss of activity or selectivity of the catalyst may also occur if the metal contaminants (e.g. Ni, V) from the hydrocarbon feedstock deposit onto the catalyst. These metal contaminants are not removed by standard regeneration (burning) and contribute to high levels of hydrogen, dry gas and coke, and reduce significantly the amount of gasoline that can be made.
U.S. Pat. No. 4,192,770 describes a process of restoring selectivity of cracking catalysts which are contaminated with metals during catalytic cracking operations. The catalysts are restored by adding boron to either the fresh make-up catalyst or to the catalyst during operations. One problem with this approach is that boron is directly placed on the catalyst, which may negatively impact the catalyst material. In addition, such an approach addresses the problem after it has occurred, by treating the catalyst after it has been contaminated. U.S. Pat. No. 4,295,955 utilizes a similar approach by restoring catalyst that has been contaminated with metals. U.S. Pat. No. 4,295,955 also shows in the examples that fresh catalyst can be treated with boron to attenuate residual metals on the fresh catalyst that contribute to the undesirable yield of hydrogen. U.S. Pat. Nos. 5,5151,394 and 5,300,215 disclose catalyst compositions comprising molecular sieve materials and a boron phosphate matrix. The Examples state that the addition of boron phosphate to the matrix does not change the physical properties or attrition resistance, but the addition of boron phosphate produced gasoline with higher octane in a cracking process.
While the aforementioned patents show the utility of boron compounds for treating contaminated catalysts and attenuating residual metals on catalyst materials, it would be desirable to provide materials that allow the addition of boron to FCC processes and units under dynamic and varying conditions. It also would be desirable to provide FCC processes and FCC catalyst compositions that can reduce coke and hydrogen yields for a variety of FCC unit conditions and hydrocarbon feeds, for example, feeds containing high levels of transition metals, such as resid feeds.